Characterization of a population of tyrosine hydroxylase-containing interneurons in the external plexiform layer of the rat olfactory bulb.

The olfactory bulb (OB) of mammals contains the major endogenous dopamine-producing system in the forebrain. The vast majority of dopaminergic neurons consists of juxtaglomerular cells, which innervate the olfactory glomeruli and modulate the entrance of sensory information to the OB. Although dopaminergic juxtaglomerular cells have been widely investigated, the presence of dopaminergic interneurons other than juxtaglomerular cells has been largely unexplored. In this study, we analyze a population of tyrosine hydroxylase (TH)-containing interneurons located in the external plexiform layer (EPL) of the rat OB. These interneurons are GABAergic and morphologically heterogeneous. They have an axon and two to four dendrites running throughout the EPL. Frequently, they have appendages similar to spines in the dendrites and, sometimes, the distal portions of the dendritic branches show enlargements or swellings similar to varicosities. Contrary to other interneurons of the EPL, the TH-containing ones do not form dendro-dendritic synapses on principal cells and do not receive dendro-dendritic synapses from them. In fact, no synapses were found from the dendrites of these interneurons. When their dendrites are involved in synaptic contacts, they are always the postsynaptic element. They receive symmetrical and asymmetrical synapses from GABAergic and non-GABAergic axons of unidentified origin. Our data indicate that the local circuits of the EPL are more complex than previously thought. Although most of the interneurons of this layer establish dendro-dendritic synaptic relationships with principal cells, the TH-containing interneurons constitute an exception to this rule, resembling interneurons from other cortical areas.

Synaptic connectivity of serotonergic axons in the olfactory glomeruli of the rat olfactory bulb.

Although the major mode of transmission for serotonin in the brain is volume transmission, previous anatomical studies have demonstrated that serotonergic axons do form synaptic contacts. The olfactory glomeruli of the olfactory bulb of mammals receive a strong serotonergic innervation from the dorsal and medial raphe nuclei. In the present report, we investigate the synaptic connectivity of these serotonergic axons in the glomerular neuropil of the rat olfactory bulb. Our study shows that serotonergic axons form asymmetrical synaptic contacts on dendrites within the glomerular neuropil. Analyzing the neurochemical nature of the synaptic targets, we have found that 55% of the synapses were on GABA-immunopositive profiles and 45% on GABA-immunonegative profiles. These data indicate that barely half of the contacts were found in GABA-immunonegative profiles and half of the synapses in GABA-positive dendrites belonging to type 1 periglomerular cells. Synaptic contacts from serotonergic axons on dendrites of principal cells cannot be excluded, since some of the GABA-immunonegative postsynaptic profiles contacted by serotonergic axons had the typical ultrastructural features of bulbar principal cell dendrites. Altogether, our results suggest a complex action of the serotonergic system in the modulation of the bulbar circuitry.